1. Field of the Disclosure
The present invention relates generally to power converters and, more particularly, the present invention relates to forward converters.
2. Background
Many electrical devices, such as cell phones, personal digital assistants (PDA's), laptops, etc., utilize power to operate. Because power is generally delivered through a wall socket as high voltage alternating current (ac), a device, typically referred to as a power converter, can be utilized to transform the high voltage ac input to a well regulated direct current (dc) output through an energy transfer element. Switched mode power converters are commonly used due to their high efficiency, small size, and low weight to power many of today's electronics. In operation, a switch is utilized to provide the desired output quantity by varying the duty cycle (typically the ratio of the on-time of the switch to the total switching period), varying the switching frequency, or varying the number of pulses per unit time of the switch in a power converter.
One power conversion topology is referred to as a forward converter. A forward converter may use one, two, or more active switches to apply an input voltage to the primary winding of a transformer, which may also be referred to as an energy transfer element. The single-switch forward converter uses one active switch to apply an input voltage to the primary winding of the transformer while the two-switch forward converter utilizes two active switches to apply an input voltage to the primary winding. In each type of converter, a secondary winding on the transformer produces a scaled replica of the voltage on the primary winding. The voltage on the secondary winding is rectified and filtered to become the output voltage.
A power converter may use a controller to provide output regulation to an electrical device, which may generally be referred to as a load, by sensing and controlling the output of the power converter in a closed loop. More specifically, the controller may be coupled to a sensor that provides feedback information about the output of the power converter in order to regulate the output quantity delivered to the load. The controller regulates the output quantity delivered to the load by controlling the one or more active switches to turn on and off in response to the feedback information from the sensor.
In operation, both the single switch and the two switch configuration may allow the magnetic flux of the transformer to reset, or in other words, return to a much lower value, when the active switches are off. Resetting the magnetic flux of the transformer prevents excess stored energy from saturating the transformer. The reset can be achieved by applying a reset voltage of appropriate magnitude and duration to the primary winding when the active switches are off.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.